It is known practice to design four-stroke cycle internal combustion engines to accommodate homogeneous-charge, compression-ignition (HCCI) combustion wherein light-load operation can be achieved with minimal throttling. This results in fuel economy comparable to the fuel economy of a diesel engine of the same displacement. A homogeneous charge of fuel and air is used in a HCCI engine in a manner similar to the use of air-fuel mixture in a spark-ignited engine, but the homogeneous charge is compressed to auto-ignition. The homogeneous-charge, compression-ignition engine thus has characteristics that are comparable in some respects to an engine with a diesel cycle.
The temperature of an air-fuel mixture in the combustion chamber of a homogeneous-charge, compression-ignition engine (HCCI) is high enough to initiate auto-ignition. The homogeneous air-fuel mixture is created either in the intake manifold or in the cylinder by early fuel injection and fast fuel-air mixing. A homogeneous air-fuel mixture in the intake manifold may be achieved, as in the case of a conventional auto cycle engine, by using a fuel-aspirating carburetor or by using a low-pressure fuel injection pump and nozzle. No spark ignition is necessary when the HCCI engine is operating in a specified operating region of the load and engine speed relationship.
It is known design practice also to expand the useful operating region of the load and engine speed relationship by using a hybrid ignition controller wherein spark ignition can be relied upon during operation at high loads and at high engine speeds with moderate loads. Thus, at lower loads, the engine can be operated in HCCI combustion mode with high dilution of the air-fuel mixture using a high air-fuel ratio or a high exhaust gas recirculation rate in order to limit the rate of combustion of the homogeneous air-fuel mixture. If the mixture is too rich, on the other hand, the rate of combustion becomes too fast and engine knocking or detonation may occur.
HCCI engines are characterized by minimal variation in the combustion of the air-fuel mixture since the initiation of combustion takes place throughout the entire mixture rather than at a single point from which a flame front develops. Instabilities of flame propagation are avoided.
An HCCI engine has reduced levels of nitrous oxide (NO.sub.x) in the exhaust gases. This is due to the low combustion temperature of the diluted mixture. It is characterized also by reduced soot or particulates in the emission due to the premixed lean mixture.
The thermal efficiency of an HCCI engine is higher than the thermal efficiency typically associated with spark ignition engines of known designs. This is due to the high compression ratio that can be used. It is due also to the unthrottled operation of the air-fuel mixture at the intake manifold, which reduces engine pumping losses. High specific heat ratios, reduced radiation heat loss and reduced cycle-to-cycle variations in combustion are further characteristics of HCCI engines where combustion does not rely upon in-cylinder air flow conditions.
The limitations of HCCI engines of known design relate to the control of the timing of the auto-ignition event and the combustion rate for the air-fuel mixture in the combustion chamber. Since combustion begins with auto-ignition of a premixed air-fuel mixture, the ignition may occur at any time during the compression process. If the engine load increases, auto-ignition tends to advance and the combustion rate tends to increase due to the rich mixture characteristic of an increased load. Thermal efficiency may decrease due to early heat release before top dead center. This results in roughness of the engine due to rapid and early combustion. NO.sub.x emissions also increase due to increased burnt gas temperature of the less diluted mixture. When the engine load decreases, on the other hand, auto-ignition tends to be retarded, which may result in misfire.
Although it is possible to control auto-ignition by changing the temperature of the air-fuel mixture at the intake manifold using electrical heater devices to promote auto-ignition, such heaters are impractical for high volume production engines used in the automotive industry.